The present invention relates to a high power doped fiber amplifier including an rare earth-doped fiber pre-amplifier (low noise) stage and a high power rare earth-ytterbium co-doped, multiple-tap output stage.
In the last decade, rare-earth doped fiber amplifiers in general, and erbium doped fiber amplifiers (EDFAs) in particular, have been extensively used in optical communication systems as a means to amplify weak optical signals between telecommunications links, particularly signals at or near the 1550 nm wavelength. Various designs of these amplifiers have been proposed to provide efficient performance, where efficiency is measured in terms of high optical gain, low noise figure, high output power and pump efficiency. Recently, with the use of EDFAs in applications such as multiple WDM systems and analog CATV systems, high optical power (along with low noise) has become essential in order to overcome the splitting losses and to have relatively high optical power at the receivers. High power levels can be achieved by increasing the pump power near the 980 nm or 1480 nm wavelengths. However, the semiconductor lasers conventionally used to emit at these wavelengths are problematic in terms of power scalability and overall lifetime.
As an alternative to providing an increased power for these newer applications, co-doping of the fiber amplifier has been proposed, where in most cases a co-doping of Er+3 and Yb+3 is used. Such a co-doping increases the amount of pump absorption and offers a flexibility in selection of the pump wavelength, owing to the broad absorption band of Yb+3 (from 800 to 1100 nm) in glass. In glass fibers which contain phosphorus, ytterbium can absorb pump power available from diode-pumped Yb or Nd-doped laser sources near 1064 nm and efficiently transfer the energy to erbium ions for power application near 1550 nm. To date, several fiber amplifiers with Er+3-Yb+3 co-doping that are pumped with a 1064 nm Yb or Nd-cladded pumping lasers have been demonstrated with co-, counter-, or side-pumping schemes. When using these amplifiers in WDM or CATV systems, the output power is split into many channels by means of a coupler (e.g., 1xc3x978, 1xc3x9716 power splitters). However, problems exist with these arrangements that limit the amplifier""s performance. In particular, allowing the propagation of a high power signal in a single optical fiber causes nonlinearities (such as four-wave mixing) that reduce the performance of the amplifier. Also, the amplifier output power is directly reduced by the splitting losses associated with the following splitter. Further, in situations involving even higher power levels (for example, a few watts) specially-designed optical components (e.g., high power connectors, couplers) may be required.
Thus, a need remains in the prior art for providing an optical amplifier design that is useful in high power, multiple output applications such as CATV and WDM systems.
The need remaining in the prior art is addressed by the present invention, which relates to a high power doped fiber amplifier including a rare earth-doped fiber pre-amplifier (low noise) stage and multiple-tap high power rare earth-ytterbium co-doped output stage.
In accordance with the present invention, the multiple-tap high power output amplifier stage comprises a plurality of concatenated sections of co-doped fiber, each separately amplified. An amplified output signal is thus tapped off of each region where two contiguous doped fibers are joined.
In one embodiment of the present invention, the concatenation region comprises a pair of multiplexers and an isolator, used to remove only the amplified information signal and allow most of the pump signal to remain within the doped fiber as it is coupled to the next stage.
In a preferred embodiment, both a co-propagating pump signal and counter-propagating pump signal are applied as inputs to the string of concatenated co-doped fiber sections.
Other and further embodiments of the present invention will become apparent during the course of the following discussed and by reference to the accompanying drawings.